In 3GPP Release 5, High-Speed Downlink Packet Access (HSDPA) is introduced, where a new modulation scheme (16 QAM) is introduced in addition to the existing QPSK (Quadrature Phase Shift Keying) modulation scheme in Release 99.
In HSDPA systems, the UE (User Equipment) reports the Channel Quality Indicator (CQI) through uplink HS-DPCCH (High Speed Downlink Physical Control Channel) to the RBS (Radio Base station), for which a single HS-PDSCH (High Speed Physical Downlink Shared Channel) sub-frame formatted with the transport block size, number of HS-PDSCH codes and modulation corresponding to the reported or lower CQI value could be received, and for which the transport block error probability would not exceed 10%, see 3GPP TS 25.214, v 5.11.0, “Physical layer procedures (FDD)”.
In 3GPP Release 5, five bits are used to report the CQI, resulting in a CQI value range of [0, 31], where there is a one decibel resolution, see 3GPP TS 25.212 v6.9.0, “Multiplexing and channel coding (FDD)”. Among these CQI values, a CQI value of “0” is specified to be “out of range”, which means that no transport block can be selected due to bad channel quality; a CQI value of 31 is not explicitly specified but in one Ericsson contribution, 3GPP R1-02-0675, Ericsson, Motorola, “Revised CQI proposal”, Paris, France, Apr. 9˜12, 2002, the CQI value of “31” is stated to be reserved.
For the purpose of CQI reporting, the UE shall assume a total received HS-PDSCH power asPHSPDSCH=PCPICH+Γ+Δ in dB  (1)where the total received power is evenly distributed among the HS-PDSCH codes of the reported CQI value, the measurement power offset is signaled by higher layers and the reference power adjustment Δ depends on the UE category, see 3GPP TS 25.214, v 5.11.0, “Physical layer procedures (FDD)”. The power offset Γ is defined as the transmit power of HS-PDSCH divided by the transmit power of CPICH (Common Pilot Channel) in the linear scale; or on the dB scale: Γ=transmit power of HS-PDSCH—transmit power of CPICH.
Γ can preferably be set as a configurable parameter and can be configured by operators.
Therefore, UEs actually assume thatSIRHS-PDSCH=SIRCPICH+Γ+Δ in dB  (2)
The mapping between the HS-PDSCH SIR (signal to interference ratio) and CQI behind the current 3GPP design is thatCQI=4.5+SIRHS-PDSCH in dB  (3)where SIR means the total HS-DSCH SIR, summed over the number of HS-PDSCH codes, see 3GPP R1-02-0675, Ericsson, Motorola, “Revised CQI proposal”, Paris France, Apr. 9˜12, 2002.
According to equation (3) above and taking the CQI range [0, 30] into account, the reportable SIRHS-PDSCH value in the dB scale is calculated and has a range of [−4.5, 25.5] dB. Any SIR value smaller than −4.5 dB is reported as a CQI value of “0” and SIR values greater than 26.5 dB are reported as a CQI value of “30”. Thus, it is important to note that the factor of 4.5 in equation (3) is not a scaling factor. Rather, it is an offset added to the SIR such that the CQI is reported in a range that starts at 0 and not −4.5.
3GPP TSG RAN has approved the SI description on “Scope of future FDD HSPA Evolution”. One way to achieve higher peak data rates is to introduce higher-order modulation (64QAM/16QAM) for HSDPA/HSUPA (High Speed Downlink Packet Access/High Speed Uplink Packet Access). However, the 64QAM (Quadrature Amplitude Modulation) modulation scheme normally requires a very good channel quality, e.g. above 25 dB SIRHS-PDSCH if 15 codes are available, see FIGS. 1a and 1b. 
FIG. 1a is a graph showing bit rate versus SIR switch point, for QPSK towards the left of the graph and 16QAM towards the right of the graph (i.e. higher SIR switch point). Also shown on the graph are 15 variations showing the effect of changing the number of channelization codes from 1 (at the bottom, i.e. relatively low bit rate) to 15 (at the top, i.e. relatively high bit rate).
FIG. 1b is the same graph as shown in FIG. 1a, but with the plots extended to show 64QAM on the right- and uppermost portion side of the graph (i.e. highest SIR switch point and highest bit rate).
As described in the foregoing, in Release 5, SIR values greater than 25.5 dB will be reported as a CQI value of 30. From FIG. 1b, it can be seen that after introducing 64QAM, obviously these high SIR values can't simply be reported as a CQI value of “30”. Otherwise, some large TBs (Transport Blocks) cannot be selected and the peak rate (−22 Mbps) cannot be reached.